Described as "the most extreme human ever discovered", the almost complete skeleton 'LB1' is of a one-metre-tall adult figure with a small skull, asymmetrical features, shortened upper leg bones, and large flat feet.

While some have argued that these unique features -- which bear some similarities to other early human species such as Homo habilis and Australopithecus afarensis -- mark LB1 to be a newly-discovered species Homo floresiensis, Professor Maciej Henneberg from the University of Adelaide, and colleagues say these features are typical of Down syndrome.

"Our consistent hypothesis sees LB1 as a developmentally abnormal member of a recent Australomelanesian H. sapiens population, its features reflecting multiple compatible causes," the authors write.

They suggest that if Down syndrome is taken into account, the skeleton is of similar height to other modern human species of the region.

Researchers had previously speculated that the skeleton's features were the result of hypothyroidism -- iodine deficiency that can lead to the developmental disorder known as cretinism.

Henneberg says this did not explain the small head size but it did provide other clues about the skeleton's pathophysiology.

"About one-third, if not 40 per cent of Down syndrome patients are hypothyroid and Down syndrome is classically related to small head circumference so it all clicked together in my head," says Henneberg, a professor of anthropological and comparative anatomy.

The team used previously unpublished measurements of LB1's head circumference to compare to the head circumference of modern adults with Down syndrome, as well as comparing limb and trunk measurements, and found a perfect match.

"Every bone characteristic fits Down syndrome and does not fit anything else."

The authors also point to the facial asymmetry of LB1, including in the wear on its teeth, as being characteristic of Down syndrome, as well as the disproportionately shortened femur in relation to the foot and arm measurements.

"There was a publication about two years ago about observations on facial asymmetries of Down syndrome patients and those fitted exactly the pattern of asymmetries on the LB1 skull," Henneberg says.

'Ignored evidence'

But Professor Colin Groves, a bioanthropologist at the Australian National University, says the papers' conclusions are "implausible".

Groves believes the authors have ignored much of the evidence in favour of LB1's status as a separate species.

"For example, the main paper makes a lot of the femur being abnormally short and they never actually show that it is abnormally short," says Groves, pointing out that the femur is in proportion with specimens from the early Pleistocene and late Pliocene to which LB1 has been previous likened.

Groves says the facial asymmetry is likely the result of damage during excavation, which has removed part of the brow ridge, nose and eye socket, rather than being symptomatic of Down syndrome.

LB1 was found alongside pieces of several other skeletons, including the jaw bone of another specimen designated LB6.

Henneberg and colleagues have described the rest of these as normal Homo sapiens, giving credence to their hypothesis that LB1's differences are due to Down syndrome.

Groves, however, says the shape of the lower jaw of LB6 is almost identical to that of LB1, and shows characteristics such as internal buttressing which are also found in ancient humans such as Homo habilis and Australopithecus afarensis.

But another of the study's authors, Robert B. Eckhardt of Pennsylvania State University, refutes Groves' criticisms.

Eckhardt says comparison of the femur to other long bones, such as the humerus and tibia, in the same skeleton demonstrate the femur is shorter.

"There is no way to compare the femur length of LB1 with other Flores specimens because no other specimen from the site has a femur preserved," says Eckhardt.

He also says the study carefully selected measurements that could not be affected by damage by excavation.

"Our measurements of facial asymmetry used features such as foramina that could not have been damaged by the rough excavation of LB1 by the original excavators, who did in fact cause damage."

Finally, Eckhardt says, the study describes several differences between the jaw of LB1 and LB6, particularly in the size and proportions of the mandibular ramus.